The present invention relates to a numerically controlled machine for processing printed circuit boards, with the machine including a workpiece holder and at least one work or operating unit that comprises a motor arbor with an exchangeable tool, and a feed mechanism with a drive motor, a control mechanism, and a movement-measuring system.
With such machines, printed circuit boards for printed circuits are processed, especially for the production of bores or holes (both throughbores and blind holes). For this purpose, the workpiece holder, upon which the printed circuit boards are secured, and the operating unit are positioned relative to one another in two coordinate directions. During the subsequent processing operation, the feed mechanism moves the arbor, along with the tool that it accommodates, in the third coordinate direction to the drilling depth. Generally, several operating units are present on a single machine in order to be able to simultaneously process several printed circuit boards or printed circuit board packets. So that for each printed circuit board different operations and holes with different diameters can be carried out, devices for automatic exchange of tools are provided, such as disclosed in German Pat. No. 33 01 036 Imhof dated Aug. 14, 1986 and corresponding to U.S. Pat. No. 4,520,551 -Imhof dated June 4, 1985.
With the heretofore known machines, the drilling depth results from the amount of feed of the motor arbor, with this amount of feed being measured by the movement-measuring system from a one-time adjusted stop or some other reference point. Due to the unavoidable tolerances in the length of the various tools, in the thickness and waviness of the printed circuit boards, and in the accuracy of the guidance between the workpiece holder and the respective operating unit, the holes cannot exactly achieve the prescribed depth. This is particularly disadvantageous when multi-layer boards are to be provided with blind holes, the depth of which, as measured from the upper surface of the boards, must be precise to within a few microns.
It is therefore an object of the present invention to improve a processing machine of the aforementioned general type in such a way that the prescribed drilling depths can be achieved within narrow tolerances in a highly reproducible manner.